1. Field of the Invention
This invention relates generally to data printers; and, more specifically to means for determining and controlling the position of printing heads in such devices.
2. Description of the Prior Art
It is known to use an elongated comb-like optical grid having openings extending along the length of the path of travel of a printing head which moves along a platen, and to use a light-emitting diode and a photo-transistor, or the like, coupled to the printing head so a signal is produced each time the printing head passes an opening in the grid. The position of the printing head along the grid can be determined by counting the number of pulses produced by the light-emitting diode and photo-transistor combination as the printing head moves laterally along the platen. However, there are practical limitations as to how closely such pulses can be spaced and how narrow their width may be made, because of the need for physically making the openings small enough and close enough together to produce more frequent pulses. Further, closer spacing typically requires more advanced fabricating technology which increases the cost of manufacture. Also, the practice of producing a single pulse for each opening passed does not indicate direction of motion of the printing head.